Method and System for Mixed Symmetric and Asymmetric Decryption of .ZIP Files

ABSTRACT

The present invention provides a method of integrating existing strong encryption methods into the processing of a .ZIP file to provide a highly secure data container which provides flexibility in the use of symmetric and asymmetric encryption technology. The present invention adapts the well established .ZIP file format to support higher levels of security and multiple methods of data encryption and key management, thereby producing a highly secure and flexible digital container for electronically storing and transferring confidential data.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of Ser. No. 10/620,960, filed Jul.16, 2003 entitled “METHOD FOR STRONGLY ENCRYPTING ZIP FILES”.

BACKGROUND OF THE INVENTION

The present invention relates generally to a method of using standard.ZIP files and strong encryption technology to securely store files, andmore particularly to a method of integrating existing strong encryptionmethods into the processing of .ZIP files to provide a highly securedata container which provides flexibility in the use of symmetric andasymmetric encryption technology. The present invention adapts the wellestablished and widely used .ZIP file format to support higher levels ofsecurity and multiple methods of data encryption and key management,thereby producing an efficient, highly secure and flexible digitalcontainer for electronically storing and transferring confidential data.

Compression of computer files has been available for many years.Compressing files can save large amounts of disk space, and can reducetransfer time when downloading files from the Internet or transferringfiles through email. Almost any file one downloads from the Internet iscompressed in some way. A standard compressed file or folder as it issometimes called contains one or more files that were compressed into asingle file or folder. Many different compression formats have beendeveloped over the years. The .ZIP format, created by the assignee ofthe present invention, is perhaps the most common compressed file formatfor the personal computer. Any file with a “.zip” extension most likelycontains one or more files of data archived, that is, each eithercompressed or stored, in the .ZIP format. “Zipping” a file has become acommonly used term meaning to compress the file into the .ZIP formatarchive so that it occupies less disk space, and similarly, “unzipping”a file means decompressing a compressed file in the .ZIP format.

A .ZIP file is generally recognized as a data compression and archivingformat invented by PKWARE, Inc. The .ZIP format is a file formatdesigned for combining data compression technology with file archivingtechniques. Many commercially available software products are availablefor compressing or “zipping” files or other data into the .ZIP format.These .ZIP files can then be used to reconstruct the original datathrough the “unzipping” process. Data compression converts the contentsof a file into an encoded format requiring less computer storage spaceor in the case of transmission less network bandwidth than the originaluncompressed file.

Archiving, in the context of a .ZIP file, is a method of storinginformation about the characteristics of a file in a catalogue of files,known as the Central Directory, inside the .ZIP file, allowing each fileto be retrieved individually by its characteristics. This capability iswidely used. These characteristics include, but are not limited to, filename, file size, and file creation date and time.

Software programs such as PKZIP® written by PKWARE, Inc. are used toprocess files in the .ZIP format. Such programs allow one or more filesof any type to be compressed and archived into a file of the .ZIP formattype for efficient file storage and transmission over computer andcommunication networks. This format and the software programs thatprocess .ZIP files have become ubiquitous.

Data encryption is used by many software programs to provide dataprivacy. Data encryption is a method of encoding data so that it cannotbe reproduced in its original form unless an associated key is provided.Decryption uses this key to convert the encrypted data back into itsoriginal state. The key is known only to the person encrypting the dataor by those other people with whom the person encrypting the datachooses to share the key. The key is used to “unlock” the data so thatit can again be used in its original form.

Keys are uniquely generated using data known to the person encrypting afile or other data associated with recipients and users of the file.This data can be a user-defined password or other random data. Severalmethods are commonly used for processing the keys used for dataencryption. Encryption using a key generated from a password is anexample of symmetric encryption. Encryption using a public/private keypair is an example of asymmetric encryption. An example of one methodfor processing encryption keys supported by this invention uses apublic/private key pair commonly associated with digital certificates asdefined by the document Internet X.509 Public Key InfrastructureCertificate and CRL Profile (RFC 2459). A digital certificate is aunique digital identifier associating a public and private key pair toan assigned individual, a group, or an organization. When used forencrypting data, the public key of an individual is used to process anencryption key which only the individual in possession of thecorresponding private key can use for decryption. A digital certificateis issued to an individual, a group, or an organization for a fixedperiod of time and can only be used during this time period. After thetime period has elapsed, the digital certificate will be considered tohave expired and must be reissued for a new time period.

The strength of a data encryption method is determined at least in partby its key size in bits. The larger the key size a data encryptionmethod uses, the more resistant it is to cryptanalysis. Cryptanalysis,or popularly “cracking”, is the unauthorized access to encrypted data.Strong encryption is a type of data encryption that uses key sizes of128 bits or more. A number of encryption encoding methods are knowntoday. Examples supported by the present invention include but are notlimited to Advanced Encryption Standard (AES), Data Encryption Standard(DES), 2DES, 3DES, and others. A number of key sizes are commonly usedtoday. Examples supported by the present invention include but are notlimited to 128 bits, 192 bits, and 256 bits.

Many software programs available today that process .ZIP files use dataencryption to encrypt files after compression as they are written to the.ZIP file. The data encryption method used by these software programsuses a key size of 96 bits or less and is considered weak or moderateencryption by today's standards. These software programs use keysgenerated using user-defined password data. Weak data encryption may notprovide sufficient security to computer users that store and transfertheir confidential data files using the .ZIP format.

Password-based key generation has been a commonly used method ofapplying data encryption, however, known vulnerabilities to crackingmethods such as “brute force password cracking” make this method ofencryption insufficient to meet today's more advanced security needs.Another known limitation of password-based security is the lack ofnon-repudiation. Non-repudiation is the ability to be certain that theperson or program that created an encrypted .ZIP file cannot deny thatfact and that their identity is bound to the .ZIP file they created.This cannot be achieved with symmetric encryption methods. Today,non-repudiation is an important aspect of security related to theimplementation of digital certificates and digital signatures. It iscritically important to be able to prove that a creator or sender of anencrypted file did in fact create the file, i.e. not repudiate his/heraction.

Therefore, a need exists to extend the options for levels of securityavailable to programs that process .ZIP files. This extended of securitycapability makes use of the encryption technologies available today orothers that may gain acceptance in the future.

SUMMARY OF THE INVENTION

The present invention provides a method of integrating multiple strongencryption methods into the processing of .ZIP files to provide a highlysecure data container which provides flexibility in the use of symmetricand asymmetric encryption technology. The present invention adapts thewell established .ZIP file format to support higher levels of securityand multiple methods of data encryption and key management, therebyproducing a highly secure and flexible digital container for storing andtransferring confidential electronic data.

The present invention provides a method of integrating multiple strongencryption methods into the processing of .ZIP files to provide a highlysecure data container which provides flexibility in the use ofencryption technology. The present invention supports existing weakencryption methods available in .ZIP software programs used today toensure backward compatibility with existing software programs that usethe .ZIP file format. Strong encryption methods are made available tocomputer users as configurable options to select when compressing andencrypting their files or other data into a .ZIP file.

The method of the present invention provides the capability of usingstrong encryption when creating .ZIP files. It is flexible in that itprovides that different encryption methods can be applied to a single.ZIP file to meet the security needs of a given computer user orapplication. Strong encryption algorithms are preferably used inconjunction with either password (symmetric) or any form ofpublic/private key (asymmetric) encryption methods. The symmetric methodpreferably includes a password defined by the user, while the asymmetricmethod preferably includes a public/private key associated with digitalcertificates to process encryption keys. The invention allows one ormore passwords and one or more public keys to be used individually, orin combination at the same time when archiving any file of any type ofdata into a secure .ZIP file. This capability is useful since secure.ZIP files are frequently distributed, or otherwise made accessible, tomultiple recipients for decryption. Some of those recipients may requirepassword access while others may require certificate access.

The method of the present invention also supports the four basicsecurity functions to be associated with encrypted files:confidentiality, message authentication, sender or creatorauthentication, and non-repudiation.

Specifically, the present invention supports non-repudiation to uniquelybind a .ZIP file with the identity of its creator, and prevent thatcreator from denying the creation of that .ZIP file. One method ofnon-repudiation used by this invention is the identity support availablewith digital signatures that can be generated using public/private keytechnology. The non-repudiation function provided by the presentinvention also preferably supports time-stamping methods for fixing thecreation of a digital signature in time, as well as time-stamped audittrails providing transaction history.

As indicated, the method of the present invention also supports messageauthentication. Message authentication ensures the data has not beenaltered since being encrypted. The present invention supports messageauthentication techniques that employ public/private key forms ofmessage authentication, as well as other methods of messageauthentication that do not require the use of public/private keys. Oneexample of an alternative method that does not use a public/private keyis a cryptographic checksum.

The method of the present invention further supports the encryption offile characteristics for each file inside a .ZIP file. Current ZIPsoftware programs encrypt only the contents of the files in a .ZIP file.The additional characteristics for each file, such as its name, size,etc., remain unencrypted. To remove the possibility that thisunencrypted data for a file could be made available to an unauthorizeduser, this information may preferably also be encrypted as an option.This additional encryption further increases the level of securityavailable to .ZIP file users.

Public keys such as those associated with digital certificates used forencrypting .ZIP file data preferably resides on a user's local computerin a file or a database, on an external device such as a Smart Card orother removable device, or in a shared data repository such as adirectory service served by an LDAP server.

The present invention also provides multiple methods of checking whethera digital certificate is valid for use. These methods preferablyinclude, but are not limited to standard methods of certificatevalidation, such as searching certificate revocation lists (CRL),certificate trust lists (CTL), and online checking via the interne usingOnline Certificate Status Protocol (OCSP) or Simple CertificateValidation Protocol (SCVP).

The method of the present invention also preferably defines data storagelocations within the established .ZIP file format specification forstoring information on the encryption parameters used when a file wasencrypted and on the keys needed when a file is to be decrypted. Onesuch example of these data storage locations includes a field toidentify that a new strong encryption method has been applied to a filein the .ZIP file. The strong encryption record will be defined within aCentral Directory storage area for each encrypted file. The CentralDirectory is a storage location defined in the .ZIP file format whichserves as a table of contents for the entire .ZIP file. An entry is madeinto the Central Directory for each file added to a .ZIP file. Adecryption record will be defined for storing the information needed toinitialize and start the decryption process. This decryption record willbe placed immediately ahead of the encrypted data for each file in a.ZIP file. This example is not the only method of storing this data asother storage methods can be defined.

The present invention provides many advantages or benefits over theprior art. One benefit is the ability to use multiple encryption methodsinstead of supporting only a single encryption method. A second benefitis the ability to use a mixture of symmetric and asymmetric encryptionin a single, secure .ZIP file. A third benefit is that the encryption ofindividual files using advanced public/private keys provides asignificantly higher level of security to computer users. A fourthbenefit is that encryption of .ZIP file data can be implemented using arange of commonly available cryptographic toolkits. A fifth benefit isthat the present invention supports using packaged or readily availableencryption algorithms to provide state-of-the-art security. A sixthbenefit is the availability of non-repudiation using digital signaturesthrough the use of public/private key technology. A seventh benefit isthat the invention ensures a high degree of interoperability andbackward compatibility by extending the current .ZIP file format.

Various other features, objects, and advantages of the invention will bemade apparent to those skilled in the art from the following detaileddescription, claims, and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a record layout of a prior art .ZIP file prior to the presentinvention.

FIG. 2 is a record layout of a .ZIP file in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, FIG. 1 shows the file format for thestandard .ZIP file, in existence prior to the present invention. FIG. 2illustrates the preferred general record layout of a ZIP file inaccordance with the present invention.

The newly modified ZIP file format specification according to thepresent invention, as published by PKWARE, Inc., is described in adocument entitled APPNOTE.TXT, which is attached hereto and incorporatedherein by reference. The new version of the .ZIP file format provides animplementation of the use of strong encryption based on a key generatedusing a password. This implementation constitutes one example of astructure and layout of the records and fields suitable for processingsecure .ZIP files as defined by the present invention. The completedescription of the conventional or standard .ZIP file format will not beincluded here since this information is generally well known. Only theportions pertaining to the new records and fields defined by the newformat, capable of storing data using strong encryption, will bediscussed in detail.

The present invention extends the original .ZIP file format with theaddition of new storage records to support the use of strong encryptionmethods including, as described above, both public/private key, orasymmetric, methods, and password-based, or symmetric, methods, and thecapability to use a mixture of symmetric and asymmetric methods.

An example of implementing a new strong encryption method is discussedbelow. This example identifies several new records and fields that mustbe defined within the .ZIP file format.

-   -   A new General Purpose Bit Flag having a hexadecimal value of        0x0040 to be set in both the Local and Central Record Headers        when strongly encrypting a file.    -   A new Decryption Header to be located immediately ahead of and        adjacent to the compressed data stored for each file.    -   A new Extra Field record definition with an ID having a        hexadecimal value of 0x0017 to be inserted into the Central        Record Header for each file.

When using these new fields for strongly encrypting files, the followingactions are indicated.

1. If the General Purpose Bit Flag value of 0x0040 is set to indicatestrong encryption was applied to a file, the General Purpose Bit Flagvalue of 0x0001 will also generally be set.

2. Files having a size of zero bytes (an empty file) should notgenerally be encrypted. As indicated, however, the file characteristicsof the archived files may be encrypted, even if the file is of zerolength and is not itself encrypted.

3. The contents of the field labeled Version Needed to Extract in boththe Local and Central Record Headers should preferably be set to thedecimal value of 50 or greater. If the AES encryption method is used,the contents of the field labeled Version Needed to Extract in both theLocal and Central Record Headers should preferably be set to the decimalvalue 51 or greater.

4. Data encryption should preferably be applied after a file iscompressed, but encryption can be applied to a file if compression isnot used. If compression is not applied to a file, it is considered tobe stored in the .ZIP file.

5. If encryption is applied using digital certificates, a list ofintended recipients will be constructed. Each entry in the recipientlist identifies a person whose public key has been used in theencryption process for a file and who is allowed to decrypt the filecontents using their private key.

Record Definitions New Decryption Header (NDH)

Size Value (bytes) Description IV size 2 Size of custom initializationvector/salt, if 0 then CRC32 + 64-bit File Size should be used todecrypt data. IV variable Initialization vector/salt (file specific)which should be used in place of CRC32 + 64-bit File Size Original Size4 Original (uncompressed) size of the following data Decryption Info.variable Decryption Information

Decryption Information (Details)

Size Value (bytes) Description Version (3) 2 Version/Format ofdecryption information. AlgID 2 Encryption Algorithm ID BitLen 2 Bitlength of the key Flags 2 Processing flags ERD size 2 Size of EncryptedRandom Data (ERD) ERD variable Encrypted Random Data Recipient Count 4Number of Recipients Hash Algorithm 2 Hash algorithm to be used tocalculate Public Key hash (absent for password based encryption) HashSize 2 Size of Public Key hash (absent for password based encryption)Recipient List variable Recipient List Element (absent for Elementpassword based encryption) Password 2 Size of random password validationdata Validation Data (Includes CRC32 of PVD; >4) MUST be size multipleof encryption block sizes Password, variable Password Validation Data(PVD) Validation Data CRC32 of PVD 4 CRC32 of PVD, used for passwordverifica- tion when decrypting data

Encryption Algorithm ID (AlgID) identifies which of several possiblestrong encryption algorithms was used for encrypting a file in the .ZIPfile. The strong encryption algorithms that can be used include but arenot limited to AES, 3DES, 2DES, DES, RC2 and RC4. The use of otherunspecified strong algorithms for encryption is supported by the presentinvention.

Hash Algorithm identifies which of several possible hash algorithms wasused for the encryption process for a file in the .ZIP file. Thealgorithms that can be used include but are not limited to MD5,SHA1-SHA512. The use of other unspecified algorithms for hashing issupported by the present invention.

Flags

The following values are defined for the processing Flags.

Name Value Description PASSWORD_KEY Ox0001 Password is usedCERTIFICATE_KEY 0x0002 Recipient List is used COMBO_KEY 0x0003 Either apassword or a Recipient List can be used to decrypt a fileDOUBLE_SEED_KEY 0x0007 Both password and Recipient List are required todecrypt a file. ERD is encrypted twice by 2 separate keys.DOUBLE_DATA_KEY Ox000f Both a password and a Recipient List are requiredto decrypt a file. File data is encrypted twice using 2 separate keys.MASTER_KEY_3DES 0x4000 Specifies 3DES algorithm is used for MSK

Recipient List Element

Size Value (bytes) Description Recipient Element size 2 Combined size ofHash of Public Key and Simple Key Blob Hash Hash Size Hash of Public KeySimple key Blob variable Simple Key Blob

New Decryption Central Record Extra Field (NDCEF)

Size Value (bytes) Description 0x0017 2 Signature of NDCEF Data Size 2Size of the following data (at least 12 bytes) Version (2) 2Version/Format of this extra field. AlgID 2 Encryption Algorithm ID.BitLen 2 Bit length of the key Flags 2 Processing flags Recipient Count4 Number of Recipients Hash Algorithm 2 Hash algorithm to be used tocalculate Public Key hash (absent for password based encryption) HashSize 2 Size of Public Key hash (absent for password based encryption)Simplified variable Simplified Recipient List Element (absent forRecipient List password based encryption) Element

Simplified Recipient List Element

Size Value (bytes Description Hash Hash Size Hash of Public Key

A simplified recipient list element is defined as a subset of arecipient list element and is stored to provide redundancy of therecipient list data for the purposes of data recovery.

Process Flow:

The following is a description of the most preferredencryption/decryption process for a single file using the storage formatdefined by this example. Any programs, software or other processesavailable to suitably perform the encryption/decryption process may beused.

Encryption:

1. Validate public/private key

2. Calculate file digital signature and time-stamp

3. Compress or Store uncompressed file data

4. Generate a File Session Key (FSK) (see below)

5. Calculate Decryption Information size

6. Adjust Compressed Size to accommodate Decryption Information andpadding

7. Save Decryption Information to .ZIP file

8. Encrypt Compressed or Stored File Data

9. Encrypt file characteristics

Decryption:

1. Decrypt file characteristics

2. Read Decryption Information from .ZIP file

3. Generate FSK (see below)

4. Verify Decryption Information (see below)

5. If Decryption Information is valid, then decrypt Compressed or StoredFile

Data

6. Decompress compressed data

7. Validate file time-stamp and digital signature

Generating Master Session Key (MSK)

1. If MASTER_KEY_(—)3DES is set, use 3DES 3-key as MSK algorithm,otherwise use specified algorithm.

2. If encrypting or decrypting with a password.

2.1.1. Prompt user for password

2.1.2. Calculate hash of the password

2.1.3. Pass calculated hash as argument into a cryptographic keyderivation function or its equivalent.

3. When encrypting using a public key(s).

3.1.1. Call a cryptographic key generation function or its equivalent togenerate random key

4. When decrypting using a private key(s).

4.1. Using Recipient List information, locate private key, whichcorresponds to one of the public keys used to encrypt MSK.

4.2. Decrypt MSK

Salt and/or Initialization Vector (IV)

1. For algorithms that use both Salt and IV, Salt=IV

2. IV can be completely random data and placed in front of DecryptionInformation

3. Otherwise IV=CRC32+64-bit File Size

Adjusting Keys

1. Determine Salt and/or Initialization Vector size of the key for theencryption algorithm specified. Usually salt is compliment to 128 bits,so for 40-bit key Salt size will be 11 bytes. Initialization Vector isusually used by block algorithms and its size corresponds to the blocksize.

2. If Salt size >0 or Initialization Vector size is >0 then set IV¹ tobe used by the specified encryption algorithm. ¹ When adjusting MSK, ifIV is smaller then required Initialization Vector (or Salt) size it iscomplimented with 0, if it is larger it is truncated. For all otheroperations IV is used as is without any modifications.

Generating File Session Key (FSK)

1. FSK SHA1(MSK(IV)). Adjust MSK with IV, and decrypt ERD (EncryptedRandom Data). Calculate hash of IV+Random Data. Pass calculated hash asargument into a cryptographic key derivation function or its equivalentto obtain FSK.

Verifying Decryption Information

1. Decryption Information contains variable length Password ValidationData (PVD).

2. First Password Validation Data Size—4 bytes are random data, and last4 bytes are CRC32 of that random data. This allows verification that thecorrect key is used and deters plain text attacks.

The following modifications are used for encrypting and decryptingmultiple files.

Multi-File Encryption:

1. Generate MSK.

2. For each file follow Encryption steps.

Multi-File Decryption:

1. Generate MSK from the file Decryption Information

2. For each file follow Decryption steps

3. If Decryption Information verification fails go to step 1

Alternate storage formats can be defined for implementing the flexiblesecurity support within ZIP files. One such alternative is to use otherfields, either existing or newly defined to denote that a strongencryption method was applied to a .ZIP archive. Another alternativecould be to use additional storage fields in addition to those definedin the above example, or to use the fields as defined, but ordereddifferently within each record. Still other implementations may usefewer, or more, records or fields than are defined by the above exampleor the records and fields may be placed in other physical locationswithin the .ZIP file.

Alternate processing methods can also be defined for implementing theflexible security support within .ZIP files. One such alternative is toimplement the encryption process for each file using anotherpublic/private key technology such as that defined by the OpenPGPMessage Format as documented in RFC 2440. Another alternative could beto use a more direct form of encryption key generation where the filesession key is directly used for encrypting each file. This method wouldnot use the indirect form described in the above example where the filesession key is derived from a master key.

While the invention has been described with reference to preferredembodiments, it is to be understood that the invention is not intendedto be limited to the specific embodiments set forth above. Thus, it isrecognized that those skilled in the art will appreciate that certainsubstitutions, alterations, modifications, and omissions may be madewithout departing from the spirit or intent of the invention.Accordingly, the foregoing description is meant to be exemplary only,the invention is to be taken as including all reasonable equivalents tothe subject matter of the invention, and should not limit the scope ofthe invention set forth in the following claims.

1-138. (canceled)
 139. A method for providing access to data in a .Zipfile format data container, said method including: receiving a datacontainer constructed in accordance with a .Zip file format, said datacontainer including: symmetric key data; asymmetric key data, whereinsaid symmetric key data and said asymmetric key data are stored in thecentral directory of said .Zip file format data container; and anencrypted data file; providing the options of: (a) receiving anasymmetric key input, and using said asymmetric key data in combinationwith said asymmetric key input to obtain a symmetric key tosymmetrically decrypt said encrypted data file to form a decrypted datafile; and (b) receiving a symmetric key input, and using said symmetrickey data in combination with said symmetric key input to obtain asymmetric key to symmetrically decrypt said encrypted data file to forma decrypted data file; and providing access to said decrypted data file.140. The method of claim 139 further including decompressing saiddecrypted data file.
 141. The method of claim 140 wherein saiddecompressing employs a Lempel-Ziv (LZ)-type data compression algorithm.142. The method of claim 140 wherein said decompressing employs aDeflate-type data compression algorithm.
 143. The method of claim 140wherein said decompressing employs a Burrows-Wheeler Transform(BWT)-type data compression algorithm.
 144. The method of claim 139wherein said encrypted data file is not compressed.
 145. The method ofclaim 139 wherein said symmetric key has a key length of at least 128bits.
 146. The method of claim 139 wherein said symmetric key has a keylength of at least 192 bits.
 147. The method of claim 139 wherein saidsymmetric key has a key length of at least 256 bits.
 148. The method ofclaim 139 wherein said symmetrically decrypting employs an AESencryption encoding.
 149. The method of claim 139 wherein saidsymmetrically decrypting employs a 3DES encryption encoding.
 150. Themethod of claim 139 wherein said asymmetrically decrypting employs aprivate key.
 151. The method of claim 139 wherein said asymmetricallydecrypting employs a X.509 digital certificate.
 152. The method of claim139 wherein at least part of said decryption key is externally input.153. The method of claim 152 wherein said at least part of saiddecryption key is a password.
 154. The method of claim 152 wherein saidat least part of said decryption key is a private key.
 155. The methodof claim 152 wherein at least part of said symmetric key is composed ofrandom data.